1999 WS owner, Poor Idle, acts in cycles

I have a 99 Windstar that is idling and running poorly.

While in idle the engine is running at about 1,200 RPM and gradually is increasing it's idle speed up to about 2,000. Once it hits it's "max" idle, something changes suddenly and the engine speed kicks back to about 800 RPM, where it is struggling to stay running. Still at idle, the RPM slowly regains itself and seems to stabilize temporarily at about 1,200. All of this cycle takes about 1.5 to 2 minutes to go through the full circle. I have watched idle through about 5 or 6 total cycles and seen the same pattern.

While being driven, a similar pattern happen where there is a total lack of power. The engine will suddenly regain power and drive about 90% normal during that period.

Can anyone point me in a good direction?

Check for vacuum leaks.Pcv hoses for leaks or collapsed rubber tees.
I CEL on

I would wonder about the Throttle Position Sensor (TPS). The TPS is located on the throttle body.....on the opposite side from the accelerator cable connection. Another thing would be a poor electrical connection. Start with the wiring around the TPS, and go from there. Look for an issue with a connector, and any place that the wiring harness may be rubbing against something. It is possible that this could not light the CEL as the computer is looking for the TPS resistance to be between a certain maximum and minimum value. It could fail.....and be giving an intermittent reading that is still "in range".

If the Check Engine Light is on.....have the codes read....you can have that done for free at some auto part stores.....like autozone.

I am experiencing a similar problem with my 98 3.8l. The engin will idle at around 1200rpm and then you hear a click and it dives to around 400 rpm and then recovers without stalling. the strange thing is it only happens when the heat or ac is switched on. also this cycle happens much quicker, about 3 seconds to cycle and repeats about every 20-30 seconds. i have recently replaced the vaccum hose comming off the intake manifold because of an air leak and now this problem has developed just in the last few days. any ideas? - thanks - drew